\begin{tikzpicture}[>=Stealth]
    \draw [->] (-1,0) -- (4,0) node[anchor=north] {$n$};
    \draw [->] (0,-1) -- (0,5.5) node[anchor=east] {$a_n$};
    \node at (-0.2,-0.2) {$O$};
    \foreach \x in {1,...,7} {
        \draw (\x/2,0.2) -- (\x/2,0) node[anchor=north] {$\x$};
    }
    \foreach \y in {1,...,10} {
        \draw (0.2,\y/2) -- (0,\y/2) node[anchor=east] {$\y$};
    }

    \foreach \n in {4,...,10} {
        \filldraw [fill=black] (\n/2 - 1.5, \n/2) circle (0.1);
    }
\end{tikzpicture}